Instrument for measuring the speed in RPM of a rotating gear

ABSTRACT

A method of and an apparatus for converting a first series of pulses into a second series of pulses having a frequency proportional to the frequency of the first series. The method and apparatus are particularly useful for providing a series of equally spaced pulses representative of the speed of a rotating gear in revolutions per minute (RPM). A proximity detector provides a pulse as each tooth of the gear traverses the face of the proximity detector and a sample period is formed by the passing of several teeth. A series of pulses is counted by a counter for the length of the sample period. The number of pulses at the end of the sample period is stored in a latch. A programmed divider counts down from the count in the latch to zero at a reference frequency and an output pulse is provided each time the programmed divider reaches zero. The programmed divider is reset to the count in the latch upon the occurrence of each output pulse. In a specific application, the number of output pulses from the counter is made equal to the number of pulses that would be generated by a gear having sixty teeth and rotating at the same speed for convenient display of RPM on a digital counter with a one-second time base.

BACKGROUND OF THE INVENTION

This invention relates to a method of and an apparatus for converting afirst series of pulses into a second series of pulses having a frequencyproportional to the first series and, more particularly, to aninstrument for measuring the velocity of a rotating element andconverting the series of pulses representative of the rotationalvelocity into a form which can be easily read by an RPM meter.

Accurate measurement of the velocity of a rotating gear is desirable.When convenient, a sixty-tooth gear is included in or connected to therotating system. A proximity detector sensing the teeth of thesixty-tooth gear is coupled to an electronic counter having a one-secondtime base. This provides a direct reading of RPM. However, if asixty-tooth gear (or multiples thereof) is not available, a variabletime base counter must be used to convert the number of pulses receivedfrom the gear into a form readable by the electronic counter having theone-second time base. Adjustment of the time base is required and theaccuracy of the system depends upon the adjustment of the time base.Adjustable time base counters are expensive and are often not readilyavailable.

I have developed a method and apparatus for converting the pulses from agear to the number of pulses which would be generated by a gear havingsixty teeth and rotating at the same velocity as the gear beingmeasured.

SUMMARY OF THE INVENTION

The present invention is directed to overcoming one or more of theproblems as set forth above.

A method of and an apparatus for converting a first series of pulsesinto a second series of pulses having a frequency proportional to thefirst series is provided. The method and apparatus are particularlyuseful in providing a series of equally spaced pulses representative ofthe velocity of a rotating gear having a known number of teeth. Aproximity detector detects the passing of gear teeth to provide a seriesof pulses. A series of pulses originating from an oscillator is countedby a counter for the length of a sample period. The length of the sampleperiod is determined by the passing of a preselected number of gearteeth past a proximity detector. At the end of the sample period, thenumber of pulses in the counter is stored in a latch. The count storedin the latch is entered into a programmed divider. The programmeddivider counts down from the count in the latch to zero at a referencefrequency. When the programmed divider reaches zero, an output pulse isprovided. Output pulses occur as the programmed divider repeatedlycounts the number stored in the latch, and the programmed divider isupdated with more recent information from the latch at the occurrence ofeach output pulse. The output may be provided to an electronic counterhaving a one-second time base to provide a direct reading of RPM.

DRAWING

FIG. 1 is a block diagram of a system illustrating the invention; and

FIG. 2 is a block diagram of the multiplier circuit of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, the velocity of a rotating gear, 10 which may haveany number of teeth, is to be measured and displayed as revolutions perminute on a digital counter 11 having a one-second time base. Thecircuit of FIG. 1 converts pulses derived from gear 10 to a series ofequally spaced pulses equivalent to those which would be generated by a"standard" gear having sixty teeth and rotating at the same speed. Asixth-tooth gear is considered standard, as the pulses derived from itwhen displayed on a digital counter having a one-second time base give areading directly in revolutions per minute. If a digital counter with atime base other than one second is used, a standard gear 10 will have adifferent number of teeth.

The passage of teeth 12 of gear 10 is detected by a proximity sensor 14producing a series of pulses having a repetition rate representing therotational velocity of the gear. Briefly, in accordance with theinvention a plurality of pulses from sensor 14 are counted to establisha sample period. The number of cycles of a reference signal which occurduring the sample period is counted and used to generate the outputsignal. The pulse conversion system is set for the number of teeth ongear 10 by setting a numeric selector 16. The system is adjusted for thedesired number of teeth per sample by selector 18. Both selectors 16 and18 may be decimal thumb wheel switches.

Oscillator 20 provides a reference frequency, F, to the divide-by-Ncounter 22 by line 24. The divide-by-N counter 22 is a programmeddivider which has a single output pulse on line 26 after the occurrenceof N number of pulses on line 24. N is entered by thumb wheel switchunit 18. The counter counts down from N and an output pulse is providedon line 26 when the divide-by-N counter 22 reaches zero. The outputpulse also resets the counter 22 to N by line 28 when it reaches zeroand the next countdown from N is started.

The output of the divide-by-N counter 22 has a frequency (F/N) and ismultiplied by multiplier 30. The value set by multiplier 30 is equal tothe proportion of T number of gear teeth 12 (as set by thumb wheelswitch unit 16) divided by reference T_(o), the number of teeth of areference gear (60, in the case of a system using a digital display witha one-second time base). Multiplication of the signal on line 26 by thisproportion generates a signal at a frequency F/N×T/T_(o), which isconnected by line 32 to counter 34. Counter 34, a 2¹⁶ bit up-counter,counts the pulses from multiplier 30 for a sample period, the length ofwhich is related to the time it takes N gear teeth to pass detector 14.The sample period begins with the movement of a tooth 12 past theproximity detector 14 and ends when N teeth, as set by thumb wheelswitch unit 18, have passed the proximity detector 14. Counter 34 countsat the frequency F/N×T/T_(o) for a period of time required for N teethof gear 10 to pass detector 14.

The output of proximity detector 14 is provided to pulse shaper 36 byline 38. Pulse shaper 36, a monostable multivibrator, assures that thepulses received from the proximity detector 14 are of even amplitude andwidth. The output of pulse shaper 36 is provided to delay circuit 40(0.5μ second delay) by line 42 and the output of the delay 40 resetsflip-flop 44 by line 46. Flip-flop 44 provides a single Q output pulseon line 49 to counter 34 upon the reception of the first pulse fromdetector 14 to start the sample period. Subsequent pulses provided tothe flip-flop 44 on line 46 during the sample period do not affect thecondition of flip-flop 44. The output of pulse shaper 36 also connectsto divide-by-N circuit 48. Divide-by-N circuit 48, a programmed divider,counts down from N to zero. N, the number of teeth per sample, isentered by thumb wheel switch unit 18. An output pulse is provided online 50 to latch 52 through delay 54 (0.5μ second delay) whendivide-by-N circuit 48 reaches zero. The occurrence of the output pulserepresents the end of the sample period. The pulse from divide-by-Ncircuit 48 causes latch 52 to store the number of pulses counted bycounter 34 at the time the pulse is received at latch 52. The occurrenceof the output pulse at 50 also resets the divide-by-N circuit 48 to N.The number of pulses in the latch 52 at the end of the sample period isdirectly proportional to the length of the sample period. Also, thelength of the sample period is a function of the selected number N andthe speed of gear 12. The higher the number N, the longer the sampleperiod. The faster the velocity of gear 12, the shorter the sampleperiod. The size of counter 34 must be sufficient to preclude anoverflow condition resulting from an extremely long sample period.

The signal from delay 54 is also provided to delay 58 (2μ second delay).The output of delay 58 sets flip-flop 44 and the Q output goes low. Thisresets counter 34 to zero and holds the divide-by-N circuit 48 andcounter 34 in a reset condition until the next pulse is received fromdetector 14. This ensures that a full sample period will be measuredinstead of the sample period being occupied by the latch pulse or thereset pulse.

The count stored in latch 52 at the end of the sample period isavailable to the programmed divider 60 by line 62. The programmeddivider 60 counts down from the count in the latch to zero and isclocked at a rate F from oscillator 20 by line 64. An output pulse isprovided on line 66 when the programmed divider 60 reaches zero. When anoutput pulse is provided on line 66, the programmed divider 60 is resetto the latest available count in latch 52. If the next sample period hasnot ended, the programmed divider 60 is provided with the same count aspreviously provided. At the end of each sample period, a new count isavailable to programmed divider 60 from latch 52. Since the programmeddivider 60 is being clocked by frequency F on line 64, F is removed fromthe equation to provide equally spaced output pulses on line 66proportional to T/T_(o) times the speed of the rotating gear 10. Outputshaper 68, a monostable multivibrator, provides pulses on output line 70to digital counter 11 which are of even amplitude and width.

Referring to FIG. 2, multiplier 30 is shown. The input of the multiplieris a series of pulses at the frequency F/N. The series of pulses on line26 is divided by divide-by-T_(o) (sixty) circuit 72 to provide F/NT_(O).The output of the divide-by-T_(o) (sixty) circuit 72 is provided tophase lock loop 74 by line 76. The phase lock loop 74 multiplies thesignal F/NT_(o) times T number of gear teeth as set by thumb wheelswitch unit 16. Programmed divider 78 counts down from T to zero andprovides an output pulse on line 80 each time zero to reached. If theoutput is to be displayed as RPM on a one-second time base, thereference divisor T_(o) is sixty. For other displays a differentreference divisor will be used.

The selection of N determines the number of gear teeth 12 which musttraverse the proximity detector 14 for a sample period. The accuracy ofthe instrument depends upon the number of pulses counted per sample. Thelarger N, the more accurate the count received and stored by latch 52.Conversely, the resolution of the instrument depends upon the number ofsample periods obtained by the instrument per revolution of gear 10. Thesmaller N, the shorter the sample period. More samples can be acquiredper revolution of the gear 10 with higher resolution. Hence, byselecting the number of teeth per sample period, the operator is allowedto select the optimum sample period to maintain the desired accuracy ofthe instrument without sacrificing resolution. The chart below providesan example of the setting of the thumb wheel switch unit 18 for gearshaving the number of teeth shown.

    ______________________________________                                        TEST GEAR TEETH  TEETH PER SAMPLE                                             ______________________________________                                        181-200          10                                                           161-180          9                                                            141-160          8                                                            121-140          7                                                            101-120          6                                                             81-100          5                                                            61-80            4                                                            41-60            3                                                            21-40            2                                                             1-20            1                                                            ______________________________________                                    

The proximity detector 14 may be of any type capable of detecting thepresence of a metal tooth. In lieu of the proximity detector 14, anoptical device may be employed if, for example, the velocity of anonmetallic gear is to be measured.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a speed measuringsystem having means for sensing rotation of a gear or the like, saidmeans generating T pulses per revolution and means including a counterwith a periodic time base for displaying the speed of rotation in termsof revolutions per unit of time, an improved circuit for converting thegenerated pulses T to a series of display counter drive pulses,comprising:a source of signal at a reference frequency; means connectedwith said sensing means for establishing a sample period of N generatedpulses T; means for deriving from said reference frequency signal aseries of pulses having a rate of directly proportional to T andinversely proportional to N; a counter for counting the derived pulseswhich occur during a sampling period; and means responsive to the countof said last mentioned counter and to said reference signal frequencyfor generating the display counter drive pulses.
 2. The circuit of claim1 including:means for selecting the value of N.
 3. The circuit of claim2 wherein the selecting means includes:a thumb wheel switch unit.
 4. Thecircuit of claim 1 wherein the means for establishing a sample periodincludes:a flip-flop coupled to the sensing means for starting thesample period upon the detection of a first generated pulse; and adivider coupled to the sensing means for ending the sample period uponthe detection of the Nth pulse.
 5. The circuit of claim 1 wherein themeans for deriving a series of pulses includes:a divide-by-sixty circuitfor dividing the reference signal by sixty to provide a divided seriesof reference pulses; and a phase lock loop for multiplying the dividedseries of reference pulses by the number of pulses per revolution. 6.The circuit of claim 5 including means for selecting the value of N. 7.The circuit of claim 6 wherein the selecting means includes a thumbwheel switch unit.
 8. The circuit of claim 1 wherein the meansresponsive to the counter includes:a latch for storing the count at theend of the sample period; a programmed divider for counting down fromthe count to zero at said reference signal frequency for providing adisplay counter drive pulse when zero is reached; and means forresetting the programed divider to the count when zero is reached. 9.The circuit of claim 8 wherein means are provided for updating the countin the latch at the end of each sample period.
 10. A circuit forconverting a first series of pulses into a second series of pulsesproportional to the first series by a predetermined ratio, said circuitincluding:a source of a first series of pulses; means connected withsaid pulse source for establishing successive sample periods of N pulseseach; means for generating a series of equally spaced reference pulseshaving a rate related to said predetermined ratio; means for countingthe reference pulses which occur during each sample period; meansoperative at the end of each sample period to store the count from thecounting means; programmed divider means for counting down from thestored count to zero at a regular rate and for providing output pulseseach time zero is reached to provide the second series of pulses; andmeans for resetting the programed divider means to the stored count uponthe occurrence of an output pulse therefrom.
 11. The pulse conversioncircuit of claim 10 wherein said means for establishing a sample periodinclude:a flip-flop responsive to the first pulse in the first series ofpulses for starting the sample period; and a divide-by-N counter whichcounts down from N upon the reception of said first series of pulses andprovides an output pulse when N pulses N in said first series of pulseshave been counted to end said sample period.
 12. The pulse conversioncircuit of claim 11 further including thumb wheel switch means forentering N into said divide-by-N counter.
 13. The pulse conversioncircuit of claim 10 wherein means for generating the series of equallyspaced reference pulses includes:a source of signal at a frequency F;means for dividing said signal by a number equal to the preselectednumber of pulses N to provide pulses at a rate F/N; and means formultiplying the pulses at the rate F/N by said predetermined ratio. 14.A circuit for converting pulses representing the rotation of a member atT pulses per revolution to a series of equally spaced output pulses atthe frequency of pulses representing rotation of said member at T_(o)pulses per revolution, comprising:a source of signal at a referencefrequency F; means for multiplying said signal at frequency F by theratio T/T_(o) ; means responsive to N pulses from the rotating memberfor establishing a sampling period; a counter responsive to saidsampling period establishing means for counting the pulses at thefrequency FT/T_(o) which occur during the sampling period; and meansconnected with said reference frequency source and said counter forestablishing a series of output pulses at a rate representing therelation of said member at T_(o) pulses per revolution.
 15. The circuitfor converting pulses of claim 14 wherein the pulses counted have afrequency of FT/NT_(o).
 16. A method of generating equally spaced pulsesrepresentative of the speed of a rotating element comprising:detecting apulse each time a tooth on the element passes a point; starting a sampleperiod when a first pulse is detected; stopping the sample period whenthe last pulse of N number of pulses is detected; generating a series ofequally spaced output pulses representative of the number of pulses thatwould be detected if a gear having sixty teeth were rotating at the samespeed as the rotating gear during the sample period; and applying theseries of equally spaced output pulses to a digital counter having aone-second time base to display the speed in RPM.
 17. The method ofclaim 16 wherein the step of generating a series of equally spacedoutput pulses includes:providing a reference signal; dividing thereference signal by a number inversely proportional to N number ofpulses; multiplying the divided reference signal by a number equal tothe number of gear teeth on the rotating gear divided by sixty toprovide a series of pulses; counting the pulses which occur during thesample period; and removing the reference signal to provide the equallyspaced output pulses indicative of the RPM of the rotating gear.